This paper presents the sulfidisation promotion effect of ammonium sulfate on the sulfidisation flotation of both malachite and copper oxide ores. Copper oxide ores are commonly difficult-to-float due to its molecular structure, and adding sodium sulfide as activator is necessary; however, its dosage to the flotation pulp must be strictly controlled, as it is also a powerful depressant for copper sulfide minerals, and will, if in excess, depress the activated copper oxide minerals. This depression will be alleviated while ammonium sulfate is added, as it accelerates the speed of sulfidisation, and alleviates excessive sulfide ions on the depression of copper oxide minerals. Ammonium sulfate, therefore, may be a sulfidisation promoter, and plays a role of sulfidisation promotion effect; it makes the coating of sodium sulfide on the ore surface more dense and stable, which is ready for xanthate adsorption on the surface of copper oxide minerals. The results of experiments show that the flotation recovery is increased 12.28% while ammonium sulfate is added in the presence of excessive sodium sulfide in the pulp, and the highest recovery is obtainable when the ammonium sulfate and sodium sulfide are in the same dosages. The sulfidisation promotion effect of ammonium sulfate is confirmed by pure copper oxide mineral flotation test and copper oxide ores flotation test, as well as by SEM images of floated malachite.